Studies on brain membrane properties are concentrated on defining the reaction mechanism of the Na, K-ATPase. This ATPase is the enzymatic representation of the plasma membrane sodium pump, responsible for the transport of Na and K out of and into cells; consequently, the enzyme not only plays a crucial role in general cell physiology, but also in nerve cells establishes and eaintains the resting membrane potential that energizes the conduction of nerve impulses. Biochemical studies are now porposed on three aspects of the enzymatic reaction process: (1) characterization of the nucleotide substrate sites, with particular emphasis on defining the enzymatic roles (catalytic and/or regulatory) of the high affinity and low-affinity substrate sites in the varigus reactions catalyzed by the ATPase and their availability in the reaction sequence; (2) description of the interactions between potassium ions and enzyme, with emphasis on those potassium sites throughwhich the phosphatase reaction is catalyzed and their relationship to sites through which such effects as nucleotide binding, typtic digestion, and Na, K-ATPase activity are influenced; (3) chemical modification of the enzyme, particularly in terms of alterations in activation by potassium ions. These experiments involve kinetic and binding studies, analyses of changes produced by chemical and enzymatic procedures, and enzyme purification and reconstitution in artifical membranes. The major goal is a descriptionof the molecular events involved in these phenomena, and their relationship to the molecular events underlying active transport.